During gas turbine operation, heavy thermo acoustic pulsations may be generated in the combustion chamber, due to an unfavourable coupling of acoustic and fluctuation of heat release rate (combustion). The risk of thermo acoustic pulsation generation is particularly high when the gas turbine is provided with lean premixed low emission combustion devices.
These pulsations act upon the hardware of the combustion device and the turbine to heavy mechanical vibrations that can result in the damage of individual parts of the combustion device or turbine; therefore pulsation must be suppressed.
In order to suppress oscillations, combustion devices are usually provided with damping devices; typically damping devices consist of quarter wave tubes, Helmholtz dampers or acoustic screens.
US2005/0229581 discloses a reheat combustion device with a mixing tube and a front plate. The front plate has an acoustic screen having holes; parallel to the acoustic screen and apart from it, an impingement plate also provided with holes, ensuing cooling of the device, is provided.
During operation, air (from a plenum containing the combustion device) passes through the impingement plate, impinges on the acoustic screen (cooling it) to then pass through the acoustic screen and enter the combustion chamber. Nevertheless this damping system has some drawbacks. In fact, cooling of the acoustic screen requires a large air mass flow, which must be diverted from the plenum into the damping volume in order to cool it.
This, in addition to reducing the damping efficiency, also increases the air mass flow, which does not take part in the combustion, such that the flame temperature increases and the NOx emissions are consequently high.